An optical coupling device which transmits a signal through transmission/reception of light is provided with a receiving circuit. The receiving circuit needs to prevent erroneous operation. The receiving circuit receives a light signal by a photodiode (PD). The photodiode converts the light signal into a photocurrent. Further, the receiving circuit converts the photocurrent into a signal voltage. The receiving circuit is provided with a comparator circuit to compare the level of the signal voltage and a threshold value so as to generate an output signal.
The level of the signal voltage changes depending on intensity of the light signal which is input into the photodiode. The luminous efficiency of a light emitting diode (LED) which is provided on a transmitting side fluctuates largely depending on temperature, and the level of the light signal changes considerably in accordance with change of an ambient temperature. As a result, the comparator may operate erroneously so that a correct signal cannot be output.
In order to prevent such an erroneous operation, a system of detecting a peak of a signal level of a signal voltage can be used so that a threshold level may follow the signal level. However, even such a system may cause an erroneous operation, due to an overshoot of a signal waveform of the signal voltage or a tailing phenomenon in which a falling portion of a signal waveform output from a photodiode is stretched. Accordingly, the receiving circuit is required to suppress erroneous operation resulting from fluctuation of a light signal.